Hard wearing surfaces are in common use in various industries, and such hard wearing surfaces are frequently obtained by coating the surface of a tool made of steel or similar metal, or other hard, enduring material, with a layer of hard wearing ceramic substance, such as carbides, nitrides and carbonitrides, or providing a hard microcrystalline diamond coating. There are known methods for obtaining hard wearing coatings, such as for example, having a coating of diamond particles in combination with a carbide or nitride layer and then filling the gaps between the abrasive particles with a softer intermetallic compound. Another known method is vapour deposition of hard-wearing ceramic materials from plasma or by utilizing molten ceramic substances. Hard wearing surfaces for use on medical, surgical and dental tools have additional requirements, as such surgical and dental tools need to be frequently sterilized, hence medical tools have to be corrosion resistant.
A device for yielding hard ceramic surfaces by cathodic are plasma deposition is described in U.S. Pat. No. 4,851,095, issued to M. A. Scobey et al. on Jul. 25, 1989. The apparatus of Scobey et al. utilizes a high intensity ion flux. Vapour deposition of a hard ceramic material, such as titanium or zirconium nitride, on a stainless steel or titanium surface by utilizing a molten evaporant and a hollow cathode, is described in U.S. Pat. No. 5,152,774, issued to W. A. Schroeder on Oct. 6, 1992. The vapour deposition of Schroeder is conducted at relatively low temperature, thus the substrate will have lost little of its initial high strength properties, however, the requirement of low surface roughness of the deposited layer is not addressed by U.S. Pat. No. 5,152,774. In U.S. Pat. No. 4,981,756, issued to H. S. Rhandhawa on Jan. 1, 1991, a method is taught to coat surgical tools and instruments by cathodic are plasma deposition. The ceramic coating obtained by this technology is a nitride, carbide or carbonitride of zirconium or hafnium, in a single layer of 3-10 μm thickness. U.S. Pat. No. 4,981,756 also refers to various publications describing known equipment for obtaining hard-wearing surfaces by cathodic are plasma deposition. U.S. Pat. Nos. 5,940,975 and 5,992,268 issued to T. G. Decker et al. on Aug. 24, 1999 and Nov. 30, 1999, respectively, teach hard, amorphous diamond coatings obtained in a single layer on thin metallic blades or similar metallic strips utilizing filtered cathodic are plasma generated by vaporizing graphite. It is noted that no interlayer is formed between the blade surface and the deposited amorphous diamond coating.
The grain size of deposits obtained in conventional cathodic plasma are methods may range between 0.5 to 10 μm. Any post-deposition heat treatment which may be required to maintain maximum hardness of the substrate's core metal, may lead to internal stresses in the coating due to differences in the grain size, and can eventually lead to abrasion, spalling, crack formation, grain separation, surface fractures, uneven edges and rough surfaces, and the like, which can drastically reduce the wear resistance and durability of surgical instruments and dental tools. None of the above discussed methods are concerned with even grain size and surface structure, and low micro-roughness of the vapour deposited hard, ceramic coatings, which have particular importance for dental and surgical tools, and in other applications where straight, sharp, even and nick-free edges are essential requirements.
Users desire cutting blades with sharp edges possessing long life and corrosion resistance. Typically, blades are initially sharpened to form a wedge shaped cutting edge and re-sharpened as needed, except in the case of razor blades which cannot be re-sharpened. Sharpness of a cutting blade is measured in terms of “ultimate tip radius”, which is different depending on the application. For kitchen knives, rotary cutters, and similar cutting instruments, ultimate tip radius may be several thousand Angstroms. In agricultural implements incorporating rotary blades that cut through the soil, axes, and in chisels, the cutting edge radius may be expressed in microns or even in millimeters rather than Angstroms. Shaving razor blades ordinarily have ultimate tip radii of about 1,500 Angstroms or less. This radius usually includes a layer of hard material coating applied to the wedge shaped base material of the razor blade. A self-sharpening blade having a cutting edge with different hardness and wear resistance on opposite sides of the blades, provided by applying different coating layers on opposite sides of the blade is described in U.S. Pat. No. 6,105,261, issued to Ecer on Aug. 22, 2000. This invention provides a solution to the problem of the cutting edge dulling by providing self-sharpening cutting edges with different hardness and wear resistance on opposite sides of the edge while both sides have micro-hardness and wear resistance significantly greater than the substrate metal. Cutting areas are kept sharp longer with this method especially in such adverse environments as in dental/surgical applications, use as saw blades and scrapers and in the construction industry. The disadvantage of this approach is that more intensive wear on one side of the edge leaves the hard layer unsupported which eventually results in a failure of the more brittle hard layer by fracturing. The soft side of the cutting edge has a higher wear rate which affects the support of the brittle thin film coating on the opposite side.
Coatings such a TiN, Ti(CN), or (TiAl)N deposited onto the blade edge region of a steel knife blade blank by a cathodic are process with simultaneous heating and rotation of the blade blank relative to the deposition sources are described in U.S. Pat. No. 5,724,868, issued to Knudsen et al. The blade edge region may be sharpened or unsharpened prior to deposition of the coating material. If the blade edge region is unsharpened prior to deposition, it is thereafter sharpened, preferably on one side only. An improvement of this method was proposed in U.S. Pat. No. 6,656,186, issued to Meckel et al. and includes depositing different coatings with different hardness on both sides of the blades adjacent to cutting edge. However, in operation the material on the softer side of the blade suffers greater wear and is not be able to support the harder coating on the opposite side of the blade. Further, this method as well as the methods described previously, does not address issues of friction and galling properties of the coated surface on the cutting tool.
It is known to coat dental tools and surgical instruments with titanium nitride and titanium, wherein the coating is obtained by conventional cathodic are deposition applied to corrosion resistant stainless steel substrates. The cutting surfaces of such medical tools need to be smooth, as well as hard-wearing to prevent trapping and retaining materials which can be harmful to the patient. Hence, another requirement is that the cutting edges be very straight, sharp and nick-free to avoid damage to the surrounding flesh and skin during dental or surgical treatment. There are known methods described, wherein the cutting tips of surgical instruments made of steel have been sand-blasted and then coated with a hard-wearing ceramic composition, however this method can, and is likely to, increase surface roughness and unevenness, rather than eliminate it. The main disadvantage of these methods is that the hard or even superhard coating with micro-hardness in excess of 20 GPa is deposited on relatively soft substrate surface made of steel or other alloy having micro-hardness less than 8 GPa. That creates a so-named egg-shell effect when the failure of the hard and brittle thin film coating is due to mechanical deformation of underlying soft substrate material.
The duplex technology utilizing ionitriding followed by thin film coating was developed to improve the wear resistance to bridge the mechanical properties between the soft substrate metal and hard coating. This technology however is limited to selective types of steels and metal alloys due to poor adhesion of the hard coatings to most ionitrided metallic materials.
In U.S. Pat. No. 6,617,057 issued to Gorokhovsky a multilayer cermet coating is described which employs alternating metal and ceramic layers. This coating architecture provides high hardness and at the same time secures necessary elasticity and ductility so the brittle hard ceramic layer will not fail due to bending and deformation of the substrates while the tool is in operation. Using the cathodic arc technology to create the multilayer coating eliminates the problems of surface roughness and increased radius of cutting edge. The coatings produced have a moderate hardness and wear resistance but exhibit relatively high friction and high galling properties. These cermet coatings have relatively higher friction in comparison with carbon diamond like (DLC) and related coatings.
There is a need for a method which can provide a fine grained, hard wearing ceramic surface that has low micro-roughness, sharp even edges, and has a low friction co-efficient and presents anti-galling properties. In preferred cases, the coating should also withstand post-deposition heat treatment without degradation of the coating.
All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the teachings of the specification.